On his blog today, Stefan Mischook shares his reasoning for making all of your pages PHP pages, regardless of static content or dynamic.

By making all your web pages PHP pages from the start, you are basically leaving open the possibility of using PHP in your web pages at any time.

By his reasoning, it provides an "out" for you later on should you decide that a page needs to have some dynamic content thrown in there somewhere (without having to change the filename - bad for SEO, supposedly).